Tomorrow, at the end of the day (June 30), there will be an extra second added to the clock, known as a ‘leap second’ and it could cause a Y2K-type problem for computers.

Leap seconds are incredibly rare and are periodically added to the official Coordinated Universal Time (UTC) to compensate for a very slight slowing in the Earth's rotation. Adding these seconds allows the world's clocks to be as accurate as possible.

Unfortunately, leap seconds present a big problem for computers which are programmed to count 60 seconds per minute, not 61.

The unexpected addition to the calendar can - and may - hobble computer systems around the world, potentially leading to outages, problems with utilities, navigation and communications systems.

What time will the leap second take place in the UK?

It happens at just before midnight UTC, which actually means just before 1am British Summer Time. So instead of ticking from 00:59:59 to 01:00:00, the clock goes to 00.59:60 before moving into the next hour.

How often do leap seconds get added?

Leap seconds were first introduced in 1972 and there have been 25 of them ever since.

The last one was in 2012 and, as expected, there were widespread problems. Major websites including Reddit, LinkedIn, Yelp, Mozilla and The Pirate Bay suffered problems. Australian airline Qantas' check-in system was also out of action for 16 hours.

Geoff Chester, public affairs officer from the US Naval Observatory in Washington estimates that around 10% of large-scale computer networks will encounter problems.

Will this one be any worse than the last one?

Potentially - particularly for the stock markets. The last leap second took place at the weekend, so companies had time to recalibrate their clocks while markets were closed. This will be the first leap second to take place during trading hours since markets went electronic, as Bloomberg points out.

Electronic trading means that transactions take place within milliseconds - so any discrepancies can mean major losses (or gains of it means acquiring information a fraction of a second before competitors).

In 2013 a timing glitch meant that Reuters published a piece of financial market information 15 thousands of a second earlier than the embargoed time of 10am. That was enough of a head start for Reuters' clients to make trades worth £18 million.

This will be a particular problem in Japan, South Korea and Australia, where markets will just be opening for trade - one second after the leap. Any glitches could be disastrous.

How do computers handle the extra second?

There are three ways: the clock can stop for a second, tick backwards or slice up the second and spread it out across more time. Amazon says it’s going to add a very small amount of time to all 86,400 seconds of the day so that it catches up by midnight.

Should I be worried?

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Unless you are responsible for running a major computer network, there’s not much you’ll be able to do about it. For most people the worst that will happen will be not being able to access web services.

Although leap seconds may even be abolished by the end of the year

The world's governments are scheduled to vote on whether to abolish the leap second at the World Radiocommunications Conference in Geneva in November this year.

Without leap seconds there would be a slip of 2-3 minutes by 2100 and around half an hour by 2700.